The separation of various hydrocarbonaceous compounds through the use of selective adsorbents is widely employed in the petroleum, chemical and petrochemical industries. Adsorption is often utilized when it is more difficult or expensive to separate the same compounds by other means such as by fractional distillation. Examples of such adsorptive separation processes include the separation of ethylbenzene from a mixture of xylenes, the separation of a particular xylene isomer such as paraxylene from a mixture of C.sub.8 aromatics, the separation of one sugar such as glucose from a mixture of two or more sugars such as glucose and fructose, the separation of acyclic olefins from acyclic paraffins and the separation of normal paraffins from non-normal hydrocarbons including isoparaffins and cyclic paraffins. The selectively adsorbed material will normally have the same number of carbon atoms per molecule as the nonselectively adsorbed materials and will have very similar boiling points, a feature which makes separation by fractional distillation very difficult. Another common application of adsorptive separation is the recovery of a particular class of hydrocarbons from a broad boiling point range mixture of two or more classes of hydrocarbons. An example of this is the separation of C.sub.10 -C.sub.14 normal paraffins from a mixture which also comprises C.sub.10 -C.sub.14 isoparaffins. My invention finds specific utility recovering straight chain (normal) paraffins from a mixture comprising normal and non-normal hydrocarbon material to yield an extract material useful for further processing as candle wax, food packaging material and detergents and, in addition, a raffinate material, now depleted of such straight chain hydrocarbon material which finds utility as light lubricating oil or diesel fuel.
The adsorptive separation of various chemical compounds is a well-developed and commercially practiced process. Representative examples of such processes are provided in U.S. Pat. Nos. 3,455,815 issued to R. G. Fickel and 4,006,197 issued to H. J. Bieser. Both of these references describe processes using molecular sieve type adsorptive compounds to separate straight chain paraffins from a mixture of isoparaffins and normal paraffins. The basic separation, operating procedures, conditions, adsorbents and feed materials described in these references are similar to those which may be employed in the subject invention. Bieser '197, in particular discloses the use of a raffinate-type sweeping agent in purification zone II to flush raffinate components from the non-selective void volume of the adsorbent, to thereby achieve increased purity of the normal paraffins in the extract. U.S. Pat. No. 4,436,533 issued to R. P. Bannon is also believed pertinent for its teaching of a process, under vapor phase conditions, for the continuous adsorptive separation of normal paraffins from non-normal paraffins in a C.sub.11 to C.sub.14 kerosene stream. U.S. Pat. No. 3,392,113 issued to A. J. De Rosset is also believed pertinent for its teaching in regard to the adsorptive separation of normal paraffins from a hydrocarbon mixture.
U.S. Pat. No. 3,715,409 issued to Broughton, discloses a process for separating aromatic hydrocarbons using an internal raffinate stream into the adsorption zone I to displace or flush desorbent material from the adsorption zone and an internal extract stream to displace raffinate material. This raffinate input stream is preferably located near the downstream boundary of the zone, i.e., near the raffinate output stream.
U.S. Pat. No. 3,053,913 issued to Norris, is believed pertinent in connection with the purification of so-called "heavy" normal hydrocarbons, that is, according to the patentee, those straight chain hydrocarbons having at least 13 carbon atoms per molecule. Therein, Norris teaches the use of a post-adsorption, pre-desorption, adsorbent bed elution step, using, as the eluent, a liquid branched chain paraffin, preferably having between four and eight carbon atoms per molecule. Norris teaches that the eluent step washes from the non-selective volume of the adsorbent (that is, the non-selective pore volume within the adsorbent particles and the interstitial volume between adsorbent particles), the residual (unadsorbed), heavy branched chain hydrocarbon material of the feed material remaining in contact with the adsorbent as a result of the adsorption step. Such residual material, if not so washed from the sieve, would become admixed with the ultimate desired product stream during the desorption step. By employing the elution step, and therefore substituting lighter, branched chain hydrocarbon material, the ultimate product of the desorption step was easily purifiable via fractional distillation or other conventional means. Thus, Norris, like Broughton and Bieser '197, disclosed a method of increasing the purity of the heavy straight chain hydrocarbon product of a conventional adsorptive separation process. In contrast, the subject invention concerns an improved process comprising a method of increasing the recovery of straight chain hydrocarbon material of a given purity.
Also believed pertinent is U.S. Pat. No. 4,021,499, issued to Bieser, which teaches, in a process for the separation of high purity ethylbenzene from a feed mixture comprising ethylbenzene and xylene isomers, a method to increase the purity of the desired raffinate product, i.e., ethylbenzene. The method comprises the recycling of a portion of desorbent-depleted raffinate material to a point within the adsorption zone of the process, thus establishing an ethylbenzene product reflux to the system. In contrast, my invention teaches a technique of introducing a wholly external, unadsorbed, non-normal hydrocarbon stream into a process, to increase the extract product recovery of normal hydrocarbons, at a point located within the adsorption zone immediately prior to the point at which feed is introduced or even, in some instances, mixed with the feed.